In a heavy saucepan, beat egg yolks with lemon juice and zest. Place under low heat. Beat in flour, 1 cup sugar, and butter. Add boiling water, cook and stir until mixture thickens. Pour into prepared pie crust. Beat egg whites until stiff, but not dry. Gradually add 3 tablespoons sugar. Spread over filling. Bake at 450 degrees until meringue is browned.